A person with a mature cataract, which significantly impairs visual function, can generally be treated by surgically extracting the impaired lens of the person and replacing it with either an intraocular lens or an extraocular lens. However, the condition cannot be addressed until properly diagnosed or determined.
Many different methods and apparatus have been developed in the past to help determine the existence or extent of a cataract. These methods and apparatus have generally made the determination based either on visual acuity tests or on an analysis of light exiting the eye of the patient. However, these may not be optimum indicators of a cataract, due to various anomalies. In the case of visual acuity tests, that depend upon light reaching the retina, the use of high contrast letters or figures may enable the patient to recognize the letters and figures and thus "pass" the visual acuity test regardless of a cataract condition.
Similarly, in another test which compares a photograph of a person's lens to a standardized series of photographs of a lens with different degrees of cataract formation in different parts of the lens, the resulting photographic images depend upon back scattered light from the lens. Because the back scattered light may not correlate highly with the location of the cataract and what the patient sees, a clinician using the photographs as the basis of an analysis will not be able to accurately determine the effect of opacities upon the patient's visual function and accordingly the patient may "pass" or may "fail" the test incorrectly. Moreover, in U.S. Pat. No. 4,863,261, issued to J. Flammer, entitled "Method of and Apparatus for Measuring the Extent of Clouding of the Lens of a Human Eye," light exiting the eye, i.e. "back scattered" light, is analyzed with respect to incident radiation to determine the extent of clouding of the lens.
Benedek et al., in U.S. Pat. No. 4,993,827 for "Method for Detecting Cataractogenesis", issued Feb. 19, 1991, collect and determine the intensity of light scattered from a measurement location in the lens and compares this value to the intensity of light scattered by a normal, clear lens to determine the degree of cataractogenesis at the specific measurement location.
Taratuta et al., in U.S. Pat. No. 5,072,731 for "Apparatus for Detecting Cataractogenesis Using Quasielastic Light Scattering", issued Dec. 17, 1991, analyze the light scattered from the lens using an autocorelation function or the power spectrum to separate the light fluctuation into two components: one caused by fast diffusing proteins and one caused by slow diffusing protein aggregates. The data is then compared to reference curves to determine the degree of cataractogenesis.
In each of the above back scattering techniques, low intensity light must be incident upon the eye in order to avoid damage to the eye. Of the low intensity incident light, a portion thereof is reflected for analysis. Because of the limited incident intensity, only a small amount of light is reflected back to a photomultiplier of limited quantum efficiency for measurement. The limited amount of reflected light and limited quantum efficiency of the photomultiplier make accurate analysis difficult.
Thus, a need exists for an improved, noninvasive, ocular disease state determination. The present invention meets this need by assessing the light that reaches the patient's retina and forms the proximal stimulus that the patient's visual system uses in the first stage of the perceptual process. The through-put quality of the axial portion of the lens is thereby measured indirectly by using the patient's visual system as a visual null indicator that enables one to track the rate of cataract formation. Use of the patient's retina itself as the detector provides a system of inherently superb quantum efficiency in contrast to that of known photomultipliers.
Use of the patient's own retina as a detector, moreover, permits the design of an instrument and a method that employs light of higher energy, of far shorter wavelength, e.g. 407 nm, during testing of the patient's eye. This shorter wavelength light, which enters the patient's eye, enables assessment of optical properties and characteristics of particles of sizes far smaller than those able to be characterized by the use of laser light of 633 nm wavelength.